In order to be able to design large photovoltaic energy generation installations in an efficient manner, it is desirable to connect as many solar modules as possible in series so that the series circuit, what is known as a string, has a very high voltage. In order not to exceed the permissible input voltage range of a connected inverter or the permissible module voltage with respect to ground potential, the length of such a string has to be limited so that the open-circuit voltage thereof does not exceed the maximum permissible value, even in the case of extreme conditions, in particular at low temperatures and high insolation.
Alternatively, it is known from the prior art to provide circuits that disconnect or short-circuit some of the modules when a maximum string voltage is exceeded in order to achieve a reduction in the voltage. After the string voltage has dropped again to a sufficient extent, the short-circuit or the disconnection can be discontinued again. One example of such a circuit is disclosed in document US 2013/0049710 A1, in which, for the purpose of voltage limitation, two or more solar modules of a string are bypassed by means of a relay when an open-circuit state of the string is identified.
Document DE 10 2010 009 120 A1, for example, further discloses a photovoltaic field of parallel strings, wherein some of the parallel strings can be short-circuited jointly by means of a short-circuiting switch, which is activated when a prescribed voltage is exceeded by the string. Since the short-circuiting of a portion of a string constitutes a loss of performance for the energy generation installations, it is desirable to be able to shorten the strings individually in order to have to shorten only the number of strings required in the present situation. In this case, however, it must be ensured that the shortened strings are not brought into a reverse current situation due to the long strings, in which reverse current situation, in the short strings, a current magnitude that significantly exceeds the rated current of the string flows in the reverse direction and can damage the strings.